docs: def set_forced_variation(self, experiment_key, user_id, variation_key):
      """ Sets users to a map of experiments to forced variations.
  
        Args:
          experiment_key: Key for experiment.
          user_id: The user ID.
          variation_key: Key for variation. If None, then clear the existing experiment-to-variation mapping.
  
        Returns:
          A boolean value that indicates if the set completed successfully.
      """
      experiment = self.get_experiment_from_key(experiment_key)
      if not experiment:
        # The invalid experiment key will be logged inside this call.
        return False
  
      experiment_id = experiment.id
      if variation_key is None:
        if user_id in self.forced_variation_map:
          experiment_to_variation_map = self.forced_variation_map.get(user_id)
          if experiment_id in experiment_to_variation_map:
            del(self.forced_variation_map[user_id][experiment_id])
            self.logger.debug('Variation mapped to experiment "%s" has been removed for user "%s".' % (
              experiment_key,
              user_id
            ))
          else:
            self.logger.debug('Nothing to remove. Variation mapped to experiment "%s" for user "%s" does not exist.' % (
              experiment_key,
              user_id
            ))
        else:
          self.logger.debug('Nothing to remove. User "%s" does not exist in the forced variation map.' % user_id)
        return True
  
      if not validator.is_non_empty_string(variation_key):
        self.logger.debug('Variation key is invalid.')
        return False
  
      forced_variation = self.get_variation_from_key(experiment_key, variation_key)
      if not forced_variation:
        # The invalid variation key will be logged inside this call.
        return False
  
      variation_id = forced_variation.id
  
      if user_id not in self.forced_variation_map:
        self.forced_variation_map[user_id] = {experiment_id: variation_id}
      else:
        self.forced_variation_map[user_id][experiment_id] = variation_id
  
      self.logger.debug('Set variation "%s" for experiment "%s" and user "%s" in the forced variation map.' % (
        variation_id,
        experiment_id,
        user_id
      ))
      return True